1. Field of the Invention
The present invention relates to a load-driving circuit for driving a load such as a resistor of airbag squib, the load-driving circuit having two transistors that are switched to be equally heated.
2. Description of Related Art
Examples of a load-driving circuit for driving a resistor of an airbag squib by means of a high side transistor and a low side transistor are shown in JP-A-10-264765, JP-A-10-297420 and JP-A-2005-88748. The driving circuit supplies current, when a collision impact is detected by an acceleration sensor, to the resistor of the airbag squib by controlling the low side transistor to a full-on control while controlling the high side transistor to a constant current control. In this case, current is set in the high side transistor and the low side transistor as shown in FIG. 18A attached hereto. A drain-source voltage Vds of the low side transistor (constituted by an LDMOSFET, for example) is almost zero because the low side transistor is used under a full-on state as shown in FIG. 18C. Therefore, influence of heat in the low side transistor is negligible. On the other hand, drain-source voltage Vds of the high side transistor is almost equal to a power source voltage as shown in FIG. 18B. Therefore, the high side transistor is considerably heated, and the heat may exceed a heat margin in the high side transistor. Accordingly, a size of the transistor has to be enlarged, or circuit components which are sensitive to heat have to be separated from the transistor as shown in FIG. 19.
To cope with the above problem, JP-A-2007-328683 proposes to switch the high side transistor and the low side transistor between the full-on control and the constant current control using a timer. However, it is required to provide a timer having a large size in the load-driving circuit.